mac80211: fix software remain-on-channel implementation

Jouni reported that when doing off-channel transmissions mixed
with on-channel transmissions, the on-channel ones ended up on
the off-channel in some cases.

The reason for that is that during the refactoring of the off-
channel code, I lost the part that stopped all activity and as
a consequence the on-channel frames (including data frames)
were no longer queued but would be transmitted on the temporary
channel.

Fix this by simply restoring the lost activity stop call.

Cc: stable@vger.kernel.org
Fixes: 2eb278e0 ("mac80211: unify SW/offload remain-on-channel")
Reported-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>